The invention relates to the field of high-performance polyamide coatings providing corrosion protection of metals, and more particularly to powder paints applied by an electrostatic method or by an immersion method. In general, polyamide powder paints undergo a more rapid electrostatic discharge than epoxy powder paints when they are in contact with the metal to be coated. This has the effect of causing detachment of some of the powder deposited electrostatically on the metal when the metal part to be coated is subjected to a shock. In addition, as soon as the powder-covered metal starts to be heated, a relaxation of the charges occurs and, depending on the nature of the object to be covered and its geometry, some of the powder may become detached before it has melted and formed the film. Moreover, in general the adhesion of polyamide coatings rapidly decreases when they are exposed to a salt fog atmosphere.
In Patent EP 969 053, the polyamide powder is modified by blending it with polyolefin waxes having a melting point of about 110 to 120° C. Thus, the powders deposited electrostatically on a metal surface become detached less easily.
The prior art has already mentioned in the following patents modified polyamide powders, but the purpose of these compositions is essentially to dispense with primer.
Patent U.S. Pat. No. 5,409,999 describes polyamide powders modified by epoxy sulphonamide resins. These resins are melt-blended with the polyamide, granulated and then ground in order to obtain a powder. This powder is then applied to an object, using an electrostatic method or using a dipping method, and is then melted in order to make a film. In the above prior art, it is explained that this powder has the advantage of avoiding the use of a primer and therefore it is possible to apply it directly to an object.
Patent U.S. Pat. No. 6,027,814 describes a technique similar to the previous one, but with a polyamide powder modified either by EVOH copolymers (ethylene/vinyl alcohol copolymers) or by ethylene/alkyl (meth)acrylate/maleic anhydride copolymers.
Patent U.S. Pat. No. 6,011,100 describes a technique similar to that of Patent U.S. Pat. No. 5,409,999 mentioned above, but with a powder modified by calcium carbonate.
It has now been found that by modifying polyamide powders by silanes, a polyamide powder paint is obtained which can be applied without an adhesion primer, considerably reducing (in the electrostatic method) the amount of powder liable to become detached from the metal when an impact is applied to the metal part.
Another advantage of this powder is that the adhesion remains constant for 2000 hours of exposure to salt fog.
The prior art has already described treatments of powders with silanes, but this was not with polyamide powders. Thus, Patent GB 1 508 587 describes the electrostatic deposition of charged glass or ceramic powders in order to make coatings on objects. These powders are firstly treated by a dihalosilane or trihalosilane in the gas phase and then the treated powder is subsequently brought into contact with steam. The dihalosilane or trihalosilane is dichlorodimethylsilane or trichloromethylsilane. It is stated on page 3, lines 54–61 that this treatment with dihalosilane or trihalosilane has the result of giving the particles thus treated a surface resistivity comparable to that of nylon (that is to say of polyamide). This therefore has nothing to do with the present invention, since its purpose is to modify polyamides.